1. Field of the Invention
The present invention relates to a film for a circuit board, especially a film for a circuit board which can be utilized in an electronic circuit and the like of portable devices such as a cellular phone, PDA (personal digital assistant), a notebook personal computer and the like, particularly a film suitable for a flexible circuit board.
2. Description of the Background
A polyimide resin film, a liquid crystal polymer film, an aramid resin film, a polyether ether ketone film and the like have found wide acceptance as insulation materials of electronic devices and the like because they are excellent in heat resistance, dimensional stability, solvent resistance and electrical and mechanical properties. Especially, a product in which a conductor layer is formed on a polyimide resin film has been often used in a flexible printed circuit board (FPC) of tape automated bonding (TAB) or the like.
With respect to a method in which a conductor layer is formed on a polyimide resin film, a laminating method in which a polyimide resin film and a copper foil are adhered with an adhesive (thermosetting resin composition) has been the most commonly used procedure. However, in a film for a board with a copper foil which is produced by this method, a hole formation step in forming a circuit is complicated. For example, when through-hole formation for connecting circuits formed on both surfaces of a film for a board is performed with a laser, it is required that a step of hole formation of copper foils on both surfaces through etching is first conducted and an insulation layer portion is then drilled with a laser. Further, in the laminating method, a copper foil having a certain thickness of, for example, 35 μm, 18 μm or 12 μm is used. There are problems that as the thickness is increased, formation of a fine circuit by a subtractive method is difficult, whereas when the thickness is decreased, the cost is increased. Especially, when an inner wall of a through-hole is plated, the thickness is increased by plating a copper foil as well, which is disadvantageous for formation of a fine circuit.
In order to solve these problems, a method in which a metallic layer as a base is directly formed on a polyimide resin film by a dry process such as sputtering or deposition and a conductor layer is then formed by electroless copper plating and further by electroplating copper (JP-A-2001-77493 and JP-A-9-136378) and a method in which a polyimide resin film is directly plated to form a conductor layer (JP-A-8-31881 and JP-A-7-243049) have been attempted. The former requires formation of a metallic layer of cobalt, nickel or chromium as a base by a dry metalizing method such as sputtering or vacuum deposition for improving adhesion between the polyimide resin film and the conductor layer. Thus, the step is complicated, and it is difficult to prevent generation of pinholes. Further, the latter also requires a step of electroless nickel plating or the like prior to electroless copper plating or electroplating copper step in addition to special hydrophilic treatment on the surface of the polyimide resin film. Thus, the step is complicated, and the adhesion strength is not necessarily satisfactory.